A wave gear device is provided with a rigid internally toothed gear, a flexible externally toothed gear, and a wave generator. The flexible externally toothed gear is flexed in the radial direction by the wave generator and made to partially mesh with the rigid internally toothed gear, and the wave generator is rotated to thereby cause the meshing position of the two gears to move in the circumferential direction and generate relative rotation between the two gears due to difference in the number of teeth of the two gears.
A typical wave generator is provided with an elliptically contoured rigid cam plate and a wave bearing mounted on the external peripheral surface of the rigid cam plate. The wave bearing is provided with a radially flexible outer ring and inner ring, and a plurality of balls or other rolling elements that are rollably mounted between the rings, and is elliptically flexed by the rigid cam plate. The wave bearing is fitted inside the flexible externally toothed gear, and the flexible externally toothed gear and the rigid cam plate are held in a rotatable state relative to each other.
The wave gear device uses the rigid cam plate of the wave generator as a rotation input component and uses the rigid internally toothed gear or the flexible externally toothed gear as a reduced-speed rotation output component. The rigid cam plate rotates while repeatedly displacing all parts of the wave bearing and the flexible externally toothed gear in the radial direction. The inner ring of the wave bearing rotates at high speed together with the rigid cam plate, and the outer ring rotates integrally with the flexible externally toothed gear, and the balls inserted between the inner and outer rings roll along a raceway of the outer and inner rings, whereby the rigid cam plate and the flexible externally toothed gear can smoothly rotate relative to each other with little torque.